Friction stir welding (FSW) is a method used to join metal workpieces. The method generally uses a cylindrical, shouldered tool with a profiled pin that is rotated at the joint line between two workpieces while being traversed along the joint line. The rotary motion of the tool generates frictional heat which serves to soften and plasticize the workpieces. This softened material, contributed by both workpieces, intermingles and is consolidated by the pin shoulder. As the pin moves laterally the frictional heating is reduced and the softened material hardens, creating a bond between the two workpieces. The best current understanding of the process is that no melting occurs and the weld is left in a fine-grained, hot worked condition with no entrapped oxides or gas porosity.
A common design of FSW stir rods is that the stirring element is substantially symmetrical with some irregularity to induce a stirring motion. Frequently the stir rod has a threaded appearance similar to a bolt. However, to promote intermingling and to retain the plasticized material in the weld zone for as long as possible the direction of rotation of the rod is such that the threads carry the plasticized material downward to create as turbulent a flow and as efficient an intermingling as possible. Particularly for metal workpieces the high thermal conductivity strongly localizes the region, which is plastic enough to be deformed by the stirring action. Thus, the width of the stirred region is substantially equal to the width of the stirring rod.